Add base_model metadata (nvidia/PixelDiT-1300M-1024px)

This repo currently has no `base_model` set in its card metadata, so it doesn't appear in any model's tree:

```yaml
base_model:
- nvidia/PixelDiT-1300M-1024px
```

Setting this will surface the model in nvidia/PixelDiT-1300M-1024px's **Model Tree** (under its quantizations/finetunes) and improve discoverability for people browsing from the base model.

Feel free to merge directly (or close).

README.md

@@ -1,4 +1,6 @@
 ---
+base_model:
+  - nvidia/PixelDiT-1300M-1024px
 license: other
 license_name: nsclv1
 tags: